I. Field of the Invention
The present invention relates generally to a hemming machine for sheet metal.
II. Description of the Prior Art
There are many previously known hemming machines for producing a sheet metal hem between two parts. Such hemming machines are frequently employed in the automotive industry as well as other industries.
These previously known hemming machines typically comprise a stationary base having a nest vertically slidably mounted to the base. The nest is dimensioned to support the part to be hemmed while actuators, typically hydraulic actuators, vertically displace the nest with its supported part.
In order to perform the hem, hemming tooling is mounted to the base and movable between an extended position and a retracted position. In its extended position, the hemming tooling overlies the nest while, conversely, in its retracted position, the hemming tooling is spaced laterally outwardly from the nest to allow the part as well as the nest to move vertically past the tooling.
Conventionally, these previously known hemming machines perform both a prehem and a final hem so that the hemming tooling contains two sets of tooling. The prehem tooling typically bends the sheet metal part to approximately 45xc2x0 while the final tooling to perform the final hem compresses the hems flatly against each other.
There have, however, been a number of disadvantages of these previously known hemming machines. A primary disadvantage is that the hydraulic actuators were required to displace the nest with its supported part between the prehem and final hem positions. Such hydraulic actuators, however, are prone to leakage and thus create workplace hazards.
A still further disadvantage of these previously known hemming machines is that, in the event that the nest and/or base deflects during the operation of the hemming machine, accurate positioning of the hemming tooling with respect to the nest is difficult to maintain. Unless the hemming tooling is accurately positioned relative to the nest, inaccuracies in the hemmed part will result.
The present invention provides a hemming machine which overcomes all of the above-mentioned disadvantages of the previously known devices.
In brief, the hemming machine of the present invention comprises a stationary base which is supported on a ground support surface. A cradle is vertically slidably mounted to the base while a nest is mounted to the cradle. The nest, in turn, supports the part to be hemmed.
Hemming tooling is also mounted to the base and movable between an extended position and a retracted position. In its extended position, the hemming tooling overlies the nest and thus the part to be hemmed. Conversely, in its retracted position, the hemming tooling is spaced laterally outwardly from the nest so that both the nest as well as the part carried by the nest can be vertically moved past the hemming tooling. Typically, the hemming tooling includes both prehem tooling which forms a substantially 45xc2x0 bend as well as the final hem tooling which flatly compresses the material around the hem together.
In order to vertically displace the cradle and thus the nest with its supported part, a single electric motor is drivingly connected to a set of two ball screws. In the preferred embodiment of the invention, a gear box is associated with each ball screw while a drive shaft extending from the electric motor is drivingly connected to each gear box so that, upon rotation of the single drive motor, the rotation of the two ball screws are automatically mechanically synchronized. Rotation of the drive motor in a first direction vertically elevates the cradle and nest while, conversely, rotation of the drive motor in the opposite direction vertically lowers the cradle and nest.
In the preferred embodiment of the invention, a two speed gear box is mechanically connected between the drive motor and the drive shaft which, in turn, is connected to the ball screws via their associated gear boxes. The two speed gear box at the output of the electric motor thus enables the cradle with its nest and supported part to be rapidly and vertically moved between the vertical positions just prior to the prehem and final hem operations. When either a prehem or final hem operation is desired, the gear box drivingly connected with the motor is switched to slow speed thus enabling the drive motor to compress the part to be hemmed against the hemming tooling at high torque in order to perform the hem.
The cradle is preferably formed by a spaced apart beam assembly having a connected portion at each end. The ball screws are threadably connected with the connected portion at each end of the cradle. This construction for the cradle thus minimizes the overall hemmer height, as well as the cradle weight and thus the required output from the drive motor while still maintaining sufficient rigidity to accurately perform the hemming operation.
In order to compensate for slight deflection of the cradle and/or base during the operation of the hemming machine, a tapered pin is connected to each set of hemming tooling. This tapered pin is received within a socket formed on the nest during upward movement of the nest from a position just prior to the hemming operation, whether prehem or final hem, and to the hemming operation. Consequently, the cooperation between the pin and socket laterally displaces the hemmling tooling relative to the base to ensure that the hemming tooling is accurately positioned with respect to the nest and thus with respect to the part supported by the nest during the prehem and final hem operation.